Panga 20 Build

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fallguy1000
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Re: Panga 20 Build

Post by fallguy1000 »

FluidDynamic wrote: Fri Apr 26, 2019 7:41 pm When using peel ply, can I prime right over the cured surface or is it too rough? Also, if I use release film instead of peel ply, will that surface be too smooth to prime?
Pp is too rough and release film will be too glassy, but peelply is a ready surface for fairing and sanding
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FluidDynamic
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Re: Panga 20 Build

Post by FluidDynamic »

I would like to put the helm/coffin cooler/storage a little further back from what the plans call for. I was thinking if building a platform that I could use as storage under a leaning post. See pic fir details. Is this doable with the center of gravity on this boat? Also, is this enough room to walk around the sides? How much width is acceptable? I can provide more measurements as well if needed?
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Last edited by FluidDynamic on Tue Apr 30, 2019 10:01 pm, edited 2 times in total.

fallguy1000
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Re: Panga 20 Build

Post by fallguy1000 »

FluidDynamic wrote: Tue Apr 30, 2019 4:35 pm I would like to put the helm/coffin cooler/storage a little further back from what the plans call for. I was thinking if building a platform that I could use as storage under a leaning post. See pic fir details. Is this doable with the center of gravity on this boat?
A Merten's question.
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TomW1
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Re: Panga 20 Build

Post by TomW1 »

FluidDynamic wrote: Tue Apr 30, 2019 4:35 pm I would like to put the helm/coffin cooler/storage a little further back from what the plans call for. I was thinking if building a platform that I could use as storage under a leaning post. See pic fir details. Is this doable with the center of gravity on this boat? Also, is this enough room to walk around the sides? How much width is acceptable? I can provide more measurements as well if needed?
You need to be very careful moving weights in the Panga20, it is a narrow, long boat. The COG is about 42% of the waterline from the transom. Jacques depending on the boat sets the COG between 40 - 44%. The narrower the boat the more critical the number is.

The thing you really need to do is figure out how much weight you are going to be moving around. Best way is to set up a spread sheet. This has been described in previous posts. As an example if you have 2 men onboard and they weigh an average of 200lbs and you move them back 2' then you are moving 200*2*2 or 800lbs effectively towards the transom. Do this for all your heavy weights, moving things forward negative. This the old fashioned way designers used to have to do it.

If you have questions, ask.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

FluidDynamic
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Re: Panga 20 Build

Post by FluidDynamic »

How much weight towards the transom beyond the center of gravity is acceptable? Isn't already weight positive towards the transom because of the motor? I guess the fuel tank under the deck in the front negates some of the weight on the transom.

fallguy1000
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Re: Panga 20 Build

Post by fallguy1000 »

FluidDynamic wrote: Wed May 01, 2019 8:33 am How much weight towards the transom beyond the center of gravity is acceptable? Isn't already weight positive towards the transom because of the motor? I guess the fuel tank under the deck in the front negates some of the weight on the transom.
I think what Tom is saying is essentially zero for static loads and I agree. Your statement above shows you are confused about center of gravity. The center of gravity already accounts for certain things and the designer knows these. You cannot move the motor, of course.. [sorry, I did not understand you were just uncertain about assumptions]

That is, static loads in the Panga must be offset across the center of gravity.

So, if you have a console that ways 100 pounds; and you move it back one foot, you have a 100 footpound moment that you must offset on the other side of the center of gravity. So, then a 30 pound battery must move 3.3 feet forward, for example.

I don't know how Mertens designed all these weights and if people moving aft are a factor, but I assume so. Mertens would know the center of gravity and the assumptions in it.

The easiest way to offset hull weights going aft is by moving the fuel tank forward. Anchor, etc.

Involving the designer is a good idea, but basically think of all the weights in lever terms.

If you move the console two feet back and the passengers two feet back, and figure them at 450; you have a 900 ft pound moment to offset. (No small task). The reason to include the designer is of you move a large fuel tank; he may tell you to add a layer of glass to the hull, etc. (example)

Another way to help make these changes possible is by building the console lighter. This is attainable with foam and vacuum, or you can use a lighter plywood, etc.
Last edited by fallguy1000 on Wed May 01, 2019 9:57 am, edited 1 time in total.
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FluidDynamic
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Re: Panga 20 Build

Post by FluidDynamic »

I understand the concepts of engineer statics. I'm a civil engineer. I just didn't know if the motor and tank we're already factored in. I guess I need to know if the COG did factor in the tank all the way forward under the step platform. If it was not, then I could use that to offset some if the weight towards the back.

pee wee
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Re: Panga 20 Build

Post by pee wee »

One of the difficulties with calculating for things like fuel tanks and bait or live wells is that they sometimes are full and sometimes not. If things are set up with an assumption of weight and it changes, so will the trim. I've read that Jacques likes to put the fuel tank under the console because it is a variable weight, unlike a battery or the console itself.

Another detail is that there are some weights that are movable, so you can do some adjusting for the occasion; many boats have a portable cooler and those could be placed forward or a passenger can move forward to help balance things. The idea is to get the fixed weights as close to balanced as you can, and be prepared to adjust as needed for the occasion.

I don't have the plans for the PG20, but there should be some mention in the notes about fuel tank and location.
Hank

fallguy1000
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Re: Panga 20 Build

Post by fallguy1000 »

FluidDynamic wrote: Wed May 01, 2019 9:36 am I understand the concepts of engineer statics. I'm a civil engineer. I just didn't know if the motor and tank we're already factored in. I guess I need to know if the COG did factor in the tank all the way forward under the step platform. If it was not, then I could use that to offset some if the weight towards the back.
Sorry if I had a tone. Mertens is the man on layout changes. The boat has a c of g and various assumptions.

What is the fuel tank size? I have a good quality aluminum from a Bayliner 22 on a shelf.
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jacquesmm
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Re: Panga 20 Build

Post by jacquesmm »

Weights in my LCG calculations are always based on a running boat: with engine, fuel, gear, expected crew etc. It would not make sense otherwise.

This is a long thread, I will read it later today and try to understand what is a concern but unless somebody makes major changes, I see no problem.

To redo my old calculations represents a lot of work. I use Orca Marine. In that software, the weight and centroids of every part is calculated and a resulting LCG is produced.
For the PG20, all that was done in an old version of Rhino and old versions of Orca. I discovered a few months ago that after I upgraded, it could not read the Orca database anymore. Which means that I have to recreate a complete set of material weights and assign all those new values. The first step is something I have to do anyway but represents about 3 days of work. The second step at least one day.
That is a lot of work and I am not ready to stop what I am doing unless it is very important.

It is quite easy to calculate changes your self if I give you the moment to trim 1" and the LCB. I should have that somewhere.
Jacques Mertens - Designer
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